Many important optical materials exhibit birefringence. Birefringence means that different linear polarizations of light travel at different speeds through the material. These different polarizations are most often considered as two components of the polarized light, one component being orthogonal to the other. Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces applied to the material.
Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam traversing the sample. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm). An “average” birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample.
Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below.
The two orthogonal polarization components described above are parallel to two orthogonal axes, which referred to as the “fast axis” and the “slow axis” of the optical material. The fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. Therefore, a complete description of the retardance of a sample along a given optical path requires specifying both the magnitude of the retardance and its relative angular orientation of the fast (or slow) axis of the sample.
The need for precise measurement of birefringence properties has become increasingly important in a number of technical applications. For instance, it is important to specify linear birefringence in optical elements that are used in high-precision instruments employed in semiconductor and other industries.
Moreover, some applications require that the retardation measurements be made across the surface of large-format optical elements or samples. For example, a manufacturer may wish to examine the retardance across the area of a large sheet of such material, thereby to determine whether the material is satisfactory (from a birefringence standpoint) before incurring further expense in processing the panel into a plurality of units.
The measurement of the birefringence across such large-format samples raises problems relating to the precise handling of the sample and instrumentation that is employed for such measurement. For example, it is impractical to move such large-format samples relative to the birefringence measurement instrument. Instead, the necessary optical components of the system can be moved relative to a stationary sample. One problem that arises with such a system is the need to ensure that components of the birefringence measurement system move precisely relative to one another and relative to the sample, thereby to provide consistently accurate birefringence measurement data irrespective of the amount the system components need to be moved in traversing large-format samples.
As noted above, external forces acting on the optical element or sample can induce birefringence. Such forces arise, for example, when a sample is bent or otherwise stressed while being held. The mass of the sample can induce some birefringence as a result of gravitational force, especially in instances where the sample is oriented with a significant amount of its mass vertically aligned. Thus, accurate measurement of the intrinsic birefringence of large-format samples requires that the optical element or sample of concern be held or supported in a manner that does not induce birefringence in the sample, which would produce an erroneous measure of the intrinsic birefringence. Specifically, such support requires that a flat sample be substantially uniformly supported in a plane without stress applied to the sample.
In addition to the need for adequately supporting the sample in a plane, the mechanism for supporting the sample must permit the passage of a light beam through the sample without interfering with that beam. The unhindered passage of a light beam through the sample and to an associated detection assembly is a critical aspect of accurate birefringence measurement. Moreover, it is most often desirable to measure the birefringence of a sample at closely spaced locations across the area of the sample. The design for a large-format sample holder, therefore, must strike a balance between adequately supporting the sample to prevent stress-induced birefringence, while still presenting a large area of the sample to the unhindered passage of light for birefringence measurement.
Of course, the ease and cost of manufacture, as well as the requirements for shipping and assembling a birefringence measurement system that includes a large-format sample holder are also important design considerations.